1. Field of the Invention
The present invention relates to a method for producing an aqueous dispersion of thermoplastic resin microparticles used in printing materials such as toner for electrophotography and ink, coating compositions, adhesives, and adhesive materials, and in textile processing, paper making/paper processing, and civil engineering, and relates to an toner for electrophotography containing thermoplastic resin microparticles obtained by the method.
2. Description of Related Art
Methods of producing aqueous dispersions of thermoplastic resin microparticles are roughly classified into polymerization granulation methods of forming emulsion particles from a polymerized monomer in a polymerization process and dispersion granulation methods of forming a resin into microparticles.
With respect to the former polymerization granulation methods, although seed polymerization methods and dispersion polymerization methods have been developed based on an emulsion polymerization technique and a suspension polymerization technique, other components such as emulsifiers and suspension stabilizers are required to stably disperse a monomer into an aqueous phase according to the objective particle diameter and it is difficult to remove them. Also, as a matter of course, this granulation method can be applied only in the production of a vinyl resin emulsion.
On the other hand, the latter dispersion granulation methods are widely applied, and for example, can be applied not only to a vinyl resin, but also to a polyaddition resin, a polycondensation resin and a natural resin, and thus manufacturing methods such as spray drying methods and coacervation methods are developed.
Phase inversion emulsification methods are one type of these dispersion granulation methods and there are known methods for producing microcapsules, comprising mixing an organic phase, which is obtained by dispersing or dissolving a hydrophobic substance into a self-water-dispersible resin dissolved in an organic solvent, with water, thereby causing phase inversion emulsification, and removing the organic solvent (see Japanese Unexamined Patent Application, First Publication, No. Hei 03-221137 (claims and pages 3 to 6); methods for producing encapsulated toners, comprising dispersing a colorant in an organic solvent solution of an anionic self-water-dispersible resin, thereby neutralizing the resin, and performing phase inversion emulsification of the resin into an aqueous medium, followed by removal of the organic solvent and further drying (see Japanese Unexamined Patent Application, First Publication, No. Hei 05-066600 (claims and pages 6 to 7)); methods for producing toners, comprising using a polyester resin having a neutralized salt structure as the anionic self-water-dispersible resin (see Japanese Unexamined Patent Application, First Publication, No. Hei 08-211655 (claims and pages 4 to 6)); and methods for producing toners for electrophotography, comprising emulsifying an organic solvent solution containing a colorant and a self-water-dispersible resin and an aqueous medium using a continuous emulsifying dispersing machine, followed by removal of the organic solvent and further drying (see Japanese Unexamined Patent Application, First Publication, No. Hei 09-297431 (claims and pages 3 to 6)). According to these methods, since the self-water-dispersible thermoplastic resin is used, an aqueous dispersion of thermoplastic resin microparticles can be produced without using auxiliary materials such as emulsifiers and suspension stabilizers.
As described above, although phase inversion emulsification methods are useful, and can be applied to thermoplastic resins, these methods were developed taking into account the preparation of the organic solvent solution of the self-water-dispersible thermoplastic resin. Therefore, only research about a combination of the self-water-dispersible thermoplastic resin and the organic solvent capable of dissolving the thermoplastic resin (good solvent) was proposed. Thus, these methods were not applied to the combination of a self-water-dispersible thermoplastic resin and the organic solvent in which it is difficult to dissolve the thermoplastic resin (poor solvent).
The phase inversion emulsification method had a drawback in that because of the combination of the self-water-dispersible thermoplastic resin and the organic solvent capable of dissolving the thermoplastic resin (good solvent), the affinity between the self-water-dispersible thermoplastic resin and the organic solvent is high even after the self-water-dispersible thermoplastic resin was dispersed into the aqueous medium and, as a result, a high concentration of the residual solvent remains in resin particles even after removal of the organic solvent.
Furthermore, aqueous dispersions obtained by mixing a neutralized acid group-containing polyester resin, a water-soluble organic compound having a boiling point of 60 to 200° C. and water at a specific ratio are known (see Japanese Unexamined Patent Application, First Publication, No. Sho 56-088454 (pages 2, 4, and 7; corresponding to U.S. Pat. No. 4,340,519); and Japanese Unexamined Patent Application, First Publication, No. Sho 56-125432 (pages 2, 4, and 7)).
As the water-soluble organic compound having a boiling point of 60 to 200° C., these patent documents disclose an organic solvent having a boiling point equal to or higher than 100° C., which dissolves the polyester resin, and an organic solvent having a boiling point lower than 100° C., which does not dissolve the polyester resin, but none discloses or teaches anything about removal of the organic solvent from the resulting aqueous dispersion and the use of the polyester resin in combination with the organic solvent which does not dissolve the polyester resin. In the Examples, after producing an aqueous dispersion by using an organic solvent having a boiling point equal to or higher than 100° C., which dissolves the polyester resin (good solvent), the resulting aqueous dispersion is used for a coating. Even if the organic solvent is removed from the aqueous dispersion obtained in the Examples, a high concentration of the organic solvent remains in the resin particles.